The most common method of controlling any temperature, pressure, rpm, or other process parameter, where high precision and good accuracy are required, is the so-called three-mode equation: EQU Output=K.sub.1 E+K.sub.2 .intg.Edt+K.sub.3 (dE/dt)
Where:
Output=the power output to the load, intended to hold that load to the desired process conditions, PA1 E=the error signal, or difference between the actual value of the controlled parameter and the desired value, and PA1 K.sub.1, K.sub.2, K.sub.3 are constants.
This is also called the P-I-D or Proportional-Integral-Derivative method because it consists of a proportional term plus an integral term plus a derivative term. This process control method has been practiced for quite some time using both older, conventional (analog) control systems or "controllers", and, more recently, using newer digital equipment. See, for example, the inventor's article "Temperature Controllers for Plastic Machinery", Plastics Design and Processing, April, 1970, p. 36, and J. Fishbeck, "Writing P-I-D Control Loops Easily in BASIC", Control Engineer, October, 1978, p. 45.
The following considers each term of the above equation in turn.